1. Field of the Invention
This invention relates to a torque sensor, particularly to a contactless (non-contact) magnetostrictive torque sensor.
2. Description of the Related Art
The contactless magnetostrictive torque sensor generally comprises a magnetic metal film exhibiting uniaxial magnetic anisotropy (a magnetostrictive film) attached to a torque transmission shaft, and an exciting coil and detector coil installed near the magnetostrictive film. Changes in inductance owing to permeability fluctuation produced in the magnetostrictive film by the applied torque are detected by the detector coil as potential differences from which the magnitude of the applied torque is detected.
Specifically, as taught in Japanese Laid-Open Patent Application No. Hei 6 (1994)-221941 (e.g., in its FIGS. 4 to 6), the applied torque is detected by providing an excitation signal, more precisely an ac (alternating current) signal, such as a sine wave or triangular wave to the exciting coil and by sensing the magnitude and phase of the induced voltage waveform.
A torque sensor installed in a vehicle or the like is usually operated by a mono (positive or negative) power supply because such a power supply is lower in cost than a dual (positive and negative) power supply. In a torque sensor operated by a mono power supply, the reference voltage indicating the midpoint of the excitation signal or detection waveform amplitude is obtained by resistance-dividing a voltage supplied from a constant-voltage regulator or by further passing the so-obtained voltage through a separate constant-voltage regulator to convert it to a predetermined voltage.
In the torque sensor of the prior art, therefore, cases arise in which the reference voltage does not accurately indicate the midpoint of the excitation signal or detection waveform amplitude owing to error (fluctuation) occurring in the constant-voltage regulators and/or resistances. Since this makes it difficult to stably generate an excitation signal (ac signal) of large amplitude that is free of upper-lower imbalance (i.e., difference between the upper and lower sides of the amplitude divided by a reference voltage Vref as the midpoint), the conventional torque sensor has a problem of undependable torque detection accuracy. The fact that the magnitude and phase of the amplitude cannot be accurately detected unless the detection waveform midpoint is accurately indicated is another cause of undependable torque detection accuracy.
Aside from the above, a contactless magnetostrictive torque sensor of this type is taught, for instance, by Japanese Laid-Open Patent Application 2001-133337 (e.g., paragraph number 0027 and FIG. 2). This conventional torque sensor is used to detect steering torque input through a steering wheel by the operator in a vehicle electric power steering system that uses a motor to provide steering torque assistance.
Specifically, the steering torque detected by a torque sensor (70) is sent to a control means (81) of an electric power steering system and the control means (81) controls a motor (82) based on the input steering torque.
The excitation signal used by a torque sensor is generally a sine wave generated by an analog oscillator circuit, typically a Hartley or Colpitts oscillator. Such a torque sensor is easily affected by temperature changes, power supply voltage fluctuation and other such disturbances. Since the conventional torque sensor may therefore be unable to obtain a stable excitation signal (sine wave), it is liable to experience degraded torque detection accuracy.
Moreover, a torque sensor, particularly one according to the prior art discussed above that is used for detecting the torque of a vehicle torque transmission shaft, is preferably constantly monitored for abnormality of output by a control means consisting of a microcomputer, for example, so that failure of the torque sensor can be detected as soon as possible (at the most suitable time). However, the conventional torque sensor and the microcomputer or other control means used to monitor the torque sensor for abnormality thereof are constituted as separate systems (an analog signal system and a digital signal system) that operate independently at different frequencies. A delay therefore arises between the time at which the torque sensor produces an output and the time at which a microcomputer carries out troubleshooting based on the output.
On the other hand, the characteristics of the analog circuit that produces the excitation signal (particularly components thereof like the secondary bandpass filter for passing only a predetermined frequency) are liable to differ from one unit to the next owing to variance arising during production. The noise that comes to be included in the excitation signal when this happens lowers the torque detection accuracy.